Due to the limitation of physical conditions, in the mobile communication system, a wireless link has low transmission rate and high bit error rate as compared with a cable link. In order to effectively use the limited wireless channel bandwidth resources, the RObust Header compression (ROHC for short hereinafter) is introduced. The core of the ROHC is to use the information redundancy between packets of traffic stream to transparently compress and decompress information in the packet header between nodes which are connected directly. The ROHC technology is described by a RFC 3095 document of the Internet Engineering Task Force (IETF), but in the document, there is no definition of IP header compression and decompression, and therefore, in June 2004, the ROHC working group has to define one framework solely for the IP header compression and decompression in the RFC 3843, and in February 2007, the IETF revised the ROHC related document, and the revised document is RFC 4815.
In the ROHC header compression, different Profiles are used to identify different data packet streams, so that a corresponding data compression and decompression framework is applied on the data packet stream. For example, the Profile 0 identity is used to transmit IP data packet stream by the way of uncompressing, the Profile 1 identity is used to perform compression processing on RTP/UDP/IP data packet stream, the Profile 2 identity is used to perform compression processing on the UDP/IP data packet stream, the Profile 3 identity is used to perform compression processing on ESP/IP data packet stream, and the Profile 4 identity is used to perform compression processing on the IP data packet stream.
The feedback is the related state information concerning a related decompression party when performing decompression on the data packet, which is transmitted by the decompressing party to a compressing party according to the decompression result after receiving the compressed data packets and performing decompression on the data packets. The compression party performs corresponding processing according to the received feedback data, so as to compress the data packets better, accurately, and efficiently.
In the ROHC header compression, the feedback is divided into an ACK feedback (a data packet transmitted by the decompressor to the compressor when the decompressor successfully decompresses the compressed packets) and a NACK feedback (a data packet transmitted by the decompressor to the compressor when the decompressor erroneously decompresses the compressed data packet). The feedback which is transmitted by the decompressor to the compressor can also carry a feedback option, wherein, the feedback option is used to notify the compressing party of the detailed reason why the data decompression fails, so that the compressing party can specifically adjust its compression strategy, react as soon as possible, and enhance the robustness of the compression and decompression. The feedback option has a CRC option for checking the feedback packet, a REJECT option which indicates that the decompressor has not enough resources to perform data compression processing, and an SN option which indicates that the serial number carried in the feedback packet is unavailable etc.
There are many feedback options in the ROHC which are transmitted by the decompressing party to the compressing party, and the CONTEXT_MEMORY option is one of them, which represents that the decompressing party has not enough memory resources to support decompression of compressed data of some specified packet stream, and therefore, the decompressing party carries a CONTEXT_MEMORY option to the compressing party in the NACK feedback, and the compressing party performs corresponding adjustment when receiving the feedback option, so that the decompressing party can performs correct decompression.
In the RFC 3843, a method for processing the CONTEXT_MEMORY feedback option by the compressing party is defined, after receiving the CONTEXT_MEMORY option, the compressing party compresses the packet data by selecting compressed packets to make the decompressing party successfully decompress with the minimum memory space, or can stop compressing the packet stream data. However, in the RFC 3843, a compression method about how to select to make a decompressing party decompress the data packets with the minimum memory space has not been provided, and the scare of the memory resources of the decompressing party may not continue.